July 3, 2013

Ocean Food Chain Likely Disrupted By Climate Change

A new study from the University of Southern California (USC) reveals climate change may be weeding out the bacteria that form the base of the ocean's food chain by selecting certain strains for survival.

There are winners and losers in everything, including climate change. Knowing which organisms will thrive and which will perish as atmospheric carbon dioxide levels and temperatures rise globally is increasingly important to scientists. The study, published in Nature Geoscience, shows the answer to this question for nitrogen-fixing cyanobacteria -- bacteria that obtains energy through photosynthesis -- has implications for every living organism in the ocean. When certain special organisms like cyanobacteria convert inert and unusable nitrogen gas from the air into a reactive form most living beings require for survival, this is known as nitrogen-fixing. Life in the ocean could not continue for long without nitrogen fixers.

"Our findings show that CO2 has the potential to control the biodiversity of these keystone organisms in ocean biology, and our fossil fuel emissions are probably responsible for changing the types of nitrogen fixers that are growing in the ocean," said David Hutchins, professor of marine environmental biology at the USC Dornsife College of Letters, Arts and Sciences.

"This may have all kinds of ramifications for changes in ocean food chains and productivity, even potentially for resources we harvest from the ocean such as fisheries production," Hutchins said.

Two major groups of nitrogen-fixing cyanobacteria were studied by the team. The first was Trichodesmium, which forms large floating colonies big enough to see with the naked eye and makes vast "blooms" in the open ocean, and secondly, Crocosphaera, which is also very abundant but is a single-celled, microscopic organism.

Prior studies revealed these two cyanobacteria types should be some of the biggest "winners" in the climate change lottery, thriving in high CO2 levels and warmer oceans. Those studies, however, only examined one or two strains of the organisms.

The difference for this study is the unique massive culture library of strains and species at USC assembled by Associate Professor Eric Webb.

Using this library, the researchers were able to show that some strains thrive more efficiently at CO2 levels that have not been seen since the beginning of the Industrial Revolution. Other strains, however, will grow better in the future "greenhouse" Earth.

"It's not that climate change will wipe out all nitrogen fixers; we've shown that there's redundancy in nature's system. Rather, increasing atmospheric carbon dioxide changes specifically which nitrogen fixers are likely to thrive," Hutchins said. "And we're not entirely certain how that will change the ocean of tomorrow."